Check for 'nil?' before 'empty?'

This fixes test breakage on Ruby 1.9.3.
Separate runtime and development dependencies
Specify versions for dependencies

Too old or too new versions of dependencies can break a library. To
avoid this, explicitly specify versions for dependencies.
Remove boilerplate comments
Merge pull request #2 from jvirtanen/fixes/versions-for-dependencies

Specify versions for dependencies
Merge pull request #1 from jvirtanen/fixes/nil-or-empty

Check for 'nil?' before 'empty?'
Merge pull request #4 from jvirtanen/cleanups/boilerplate-comments

Remove boilerplate comments